Conventional methods of spray-coating solid powder on a substrate in a vacuum state include a vacuum plasma spray (VPS) method, a vacuum cold spray method, an aerosol deposition (AD) method and the like. In the above conventional methods, there is a systemic difficulty in feeding a certain amount of solid powder to a transport pipe smoothly and spraying the solid powder onto a substrate, and for this reason, it is difficult to form a thin or thick coating layer having a uniform thickness, and furthermore, it is difficult to form a uniform coating layer on a substrate having a three-dimensional shape.
In order to form a coating layer having uniform quality by use of a technique of spray-coating solid powder on a substrate in a vacuum state, the solid powder should be able to be fed uniformly to a transport pipe, and this uniform feeding should be able to be continuously maintained.
FIG. 6 is FIG. 1 of U.S. Pat. No. 7,153,567. As shown in FIG. 6, a conventional aerosol deposition (AD) method (U.S. Pat. No. 7,153,567, entitled “Composite structure and method and apparatus for forming the same”, hereinafter referred to as “Prior Art 1”) employs a method in which a compressed gas is supplied to a chamber containing powder to form aerosol and the aerosol powder is supplied to a transport pipe. In this method, the powder is irregularly scattered by the compressed gas, and thus it is impossible to feed the powder to the transport pipe in a uniform amount.
In an attempt to solve the problem of non-uniform feeding of solid powder, which occurs in Prior Art 1, Korean Patent No. 10-1228004 (PCT/JP2009/054344; EP2264222, entitled “Composite structure forming method, controlled particles, and composite structure forming system”; hereinafter referred to as “Prior Art 2”) discloses an improved method in which controlled particles are stored in a storage mechanism, aerosolized, and supplied to a transport pipe.
As shown in FIGS. 21 to 30 attached to the specification of Prior Art 2 (EP2264222)”, powder is uniformly fed from the controlled particle-containing storage mechanism and the uniform feeding mechanism, but as shown in FIG. 16 attached to the same specification, the powder is fed to an aerosolization mechanism, like Prior Art 1, and thus the powder reaches a state in which it can be transported in an irregular and non-uniform manner. FIG. 7 is FIG. 16 of EP2264222.
When Prior Art 1 and Prior Art 2 are applied to an apparatus of coating solid powder on a substrate in a vacuum state, the chamber or storage mechanism containing the solid powder will be in a vacuum state during operation of the coating apparatus, and thus the solid powder will be irregularly sucked into the transport pipe. For this reason, it will be difficult to feed a uniform amount of powder to the transport pipe.
FIG. 8 is FIG. 1 of U.S. Pat. No. 6,759,085. In the technology disclosed in U.S. Pat. No. 6,759,085 (entitled “Method and apparatus for low pressure cold spraying”; hereinafter referred to as “Prior Art 3”) as shown in FIG. 8, a solid powder feeder is maintained in a vacuum state during operation of the coating apparatus. For this reason, it is difficult to feed a uniform amount of powder to the transport pipe, and thus it is difficult to form a coating layer having a uniform thickness on a substrate. Particularly, it is more difficult to control the coating layer to a thickness of several micrometers.
FIG. 9 is Figure of US 2011/0104369. US 2011/0104369 (entitled “Apparatus and method for continuous powder coating”, hereinafter referred to as “Prior Art 4”) as shown in FIG. 8 is characterized in that powder feeding is achieved in a more uniform and regular manner compared to the powder feeding methods provided in Prior Art 1 to Prior Art 3.
FIG. 10 is FIG. 1 of US 2013/0192519. Korean Patent No. 10-1065271 (PCT/KR2010/006889; US 2013/0192519; entitled “Solid powder coating apparatus”; hereinafter referred to as “Prior Art 5”) is characterized in that one side of a transport pipe is opened to atmospheric pressure, and thus powder feeding is achieved in a more uniform and regular manner compared to the powder feeding method provided in Prior Art 4. However, it is required to overcome the problem in that the suction of solid powder during operation of the coating apparatus is irregular. In addition, Prior Art 5 does not employ a method of transporting solid powder by supplying a compressed gas as disclosed in the prior art, but employs a method in which an air suction unit and a solid powder feeding unit communicate with each other and a mixture of sucked air and solid powder is supplied to a transport pipe. Also, Prior Art 5 employs a method of transporting and spraying solid powder by use of a block chamber communicating with the transport pipe. In addition, in Prior Art 5, the flow rate of a carrier gas capable of flowing through the transport pipe is determined according to air suction flow rate, the cross-sectional area of a spray nozzle, the vacuum pressure of a coating chamber, and chocking conditions, and thus the pressure of the transport pipe is determined so that the solid powder is sprayed through the spray nozzle provided in the vacuum chamber. However, in Prior Art 5, there may be a case in which the spray speed of the carrier gas is not controlled only by the flow rate of air sucked. Thus, it is needed to provide a means and a method capable of controlling even spray speed which is difficult to control only by the flow rate of air sucked.
In other words, as described above, it is needed to provide an apparatus and a method for coating solid powder, which are configured such that solid powder is uniformly fed to a transport pipe, and even a very small amount of solid powder is fed continuously in a finely controlled manner, and a carrier gas is sprayed into a coating chamber at subsonic or supersonic speeds depending on the cross-sectional area of a spray nozzle and the internal pressure of the coating chamber, and the speed of the carrier gas is controlled depending on the required spray speed of solid powder, whereby the quality of coating can be maintained uniformly regardless of the kinds of solid powder and substrate, and a uniform coating thickness can be achieved.